1. Field of the Invention
This invention relates to a sheet feeding apparatus for feeding sheets sheet by sheet by a retard separation method and to an image forming apparatus and an image reading apparatus such as a photocopier or printer having the sheet feeding apparatus.
2. Description of Related Art
FIG. 19 and FIG. 20 show a schematic cross section and a drive portion illustration of sheet feeding apparatuses of two types, respectively, capable of feeding sheets successively using representative retard separation methods that have been used conventionally. First, as the first prior art, a retard separation sheet feeding apparatus having a pickup roller is described in reference to FIG. 19.
A feeding motor M for driving the apparatus has a route to transmit the drive force to a feeding roller 203 and a pickup roller 202, and a route to transmit the drive force to a separating roller 204.
An one-way clutch 211 for limiting rotation in a direction opposite to the sheet feeding direction is formed at a feeding roller shaft 213.
The separating roller 204 formed in contact with the feeding roller 203 with a prescribed pressure is supported rotatively via a torque limiter 212 to a separating roller shaft 214 rotating in a direction opposite to the sheet feeding direction.
The pickup roller 202 feeds one sheet only out, and where the sheet is nipped at a nipping portion formed at a pressurized contact portion between the feeding roller 203 and the separating roller 204, the separating roller 204 rotates in association with the rotation of the feeding roller 203 in the sheet feeding direction, but where plural sheets come into the nipping portion between the feeding roller 203 and the separating roller 204 (hereinafter, this situation is referred to as xe2x80x9cdouble feedingxe2x80x9d), the torque limiter operates to render the separating roller 204 rotates in the direction opposite to the sheet feeding direction, thereby leaving the topmost one sheet only and returning other doubly fed sheets to a stacking plate 201.
The pickup roller 202 is controlled to feed the sheets stacked on the stacking plate 201 sheet by sheet in a direction of the feeding roller 203 and to escape from the sheet surface upon operation of a pickup solenoid 206 after the sheet fed reaches the nipping portion between the feeding roller 203 and the separating roller 204.
This is for avoiding operation by the separating roller 204 to return the sheets in the direction of the stacking plate 201from being disturbed when plural sheets are fed to the nipping portion.
In this prior art, the pickup roller 202 is rotatively supported around the feeding roller shaft 213 by a pickup supporting plate 215. According to whether a pickup lever 208 moving in association with the pickup solenoid 206 is pushing up the pickup supporting plate 215 or not, the pickup roller 202 switches the pickup roller""s position for pressing to and escaping from the sheet S.
When a user sets a sheet bundle to this sheet feeding apparatus, the pickup roller 202 is structured to escape by the pickup solenoid 206 during non-operation period of the apparatus so as not to interfere the pickup roller 202 with the setting operation.
Where one sheet fed by the feeding operation and separated by the separating roller 204 is transferred to the nipping portion of the pulling-out roller pair 205 formed on a downstream side in the sheet feeding direction, a controlling means, not shown, stops drive of the feeding motor M1 until the sheet rear end passes the nipping portion between the feeding roller 203 and the separating roller 204. Where the feeding motor M1 stops drive, a stepping motor or the like is used which does not rotate in a direction opposite to the drive direction.
The reason that the drive of the feeding motor M1 is stopped is to prevent a sheet subsequent to the forgoing sheet from being fed together where the leading end of the sheet subsequent to the sheet being fed is conveyed right before the nipping portion between the feeding roller 203 and the separating roller 204.
The one-way clutch 111 is so formed that the feeding roller 203 can rotate along with the sheet until the sheet rear end passes the nipping portion even after the feeding motor M1 stops its drive.
In addition, when the user sets the sheet bundle, a shutter 210 is provided to give the user setting feeling to prevent the sheet bundle from reaching the nipping portion by mistaken control of the user, or to prevent pickup failure due to mistaken setting made by the user. The shutter 210 moves to a position to hit the sheet bundle during the non-operation period of the apparatus by a shutter solenoid 207, and during feeding operation, can escape to a position not disturbing the feeding operation.
Next, as a second prior art, a sheet feeding apparatus of a retard separation method not using any pickup roller is described using a schematic cross section shown as FIG. 20.
As shown in FIG. 20, sheets S stacked on an intermediate plate 306 in a cassette 307 are lifted together with the intermediate plate 306 by a pressing arm 308 and a sheet pressing spring 305 and is normally in pressurized contact with the feeding roller 301 to receive feeding pressure.
A separating roller 302 provides the feeding roller 301 retard pressure (separating roller pressure). In this state, if the feeding roller 301 rotates in a direction for feeding sheets, the sheet S in pressurized contact with the feeding roller 301 is fed out and reaches the nipping portion formed between the feeding roller 301 and the separating roller 302.
At that time, if the sheet S is solely nipped at the nipping portion, the separating roller 302 rotates together with the feeding roller 301 in a direction for feeding sheet by operation of a torque limiter 303 formed in a united body with the feeding roller 301, thereby feeding the sheet S.
However, if plural sheets are nipped at the nipping portion, the separation rotates in a direction to return the doubly fed sheets with a prescribed torque by operation of the torque limiter 303, thereby preventing the sheets from being doubly fed.
The two sheet feeding apparatuses using major retard separation methods are described above, but those sheet feeding apparatus suffer from the following problems.
With the sheet feeding apparatus of the first prior art, to use the retard separation feeding method, the pickup roller is necessary to feed the sheet from the sheet bundle sheet by sheet, and a escaping mechanism such as the shutter solenoid 207 and the like is required to escape the pickup roller 202.
Moreover, since the apparatus requires the mechanism made of the shutter 210 and the shutter solenoid 207, the apparatus also requires an installation space for the mechanism, and this compelled the apparatus to be larger. The mechanism further requires larger number of parts, makes the structure complicated, and renders the manufacturing costs higher.
In a meantime, there is a problem relating to paper jamming handling. Since one-way clutch 211 for limiting the rotation in the direction opposite to the sheet feeding direction is formed at the feeding roller 203, the feeding roller 203 does not rotate in the direction opposite to the sheet feeding direction. The sheets jammed at the nipping portion between the feeding roller 203 and the separating roller 204 can be pulled only on a side of the sheet feeding direction, so that it is required to do work such as removal of the apparatus to handle the paper jamming and that such a removal imposes a duty on the user. In addition, when the user forcibly pulls sheets in the direction opposition to the sheet feeding direction, the sheets may be broken, and the sheets left within the apparatus may cause further problems.
To solve the problems relating to the first prior art, the second prior art has been conceived as a retard separation method as a type not using such a pickup roller. With this structure, the feeding operation is made without using any pickup roller, so that the apparatus structure is made simple, and so that the structure is very advantageous in terms of costs and installation space.
However, with this structure, the sheets S stacked on the intermediate plate 306 in the cassette 307 are lifted together with the intermediate plate 306 by the sheet pressing spring 305 and is normally in pressurized contact with the feeding roller 301. Therefore, the intermediate plate pressure plays the major role to the feeding operation and separation operation.
The intermediate plate pressure exerted by the sheet press spring 305 may vary depending on the stacked number of the sheets in the cassette 307, so that feeding and separation conditions are different between when the sheets are fully stacked and when the sheets are stacked in a small number.
Since the sheets S are normally in pressurized contact with the feeding roller 301, the stacked sheets S always receive the intermediate plate pressure. When the user tries to return the separated doubly fed sheets to the original position, the doubly fed sheets may not be returned smoothly because the sheet bundle is nipped by the feeding roller 301 and the intermediate plate 306.
Accordingly, the returning force by the torque limiter 303 is required to be set larger in order to prevent such double feeding in this mechanism.
On the downstream side of the feeding roller 301 and the separating roller 302 in the sheet feeding direction, a conveyance roller pair is generally formed. Such a conveyance roller pair has to pull the normally pressed sheets S from the nipping portion between the feeding roller 301 and the separating roller 302, so that the load exerted to the conveyance roller pair becomes large, and so that the lifetime of the conveyance roller pair becomes shorter.
In addition, not only the intermediate plate pressure, but the torque limiter 303 formed at the separating roller 302 gives a torque in a direction opposite to the sheet feeding direction, so that the torque also gives effects as a load to the conveyance roller pair. Therefore, as the higher the torque value of the torque limiter, the larger the load exerted to the conveyance roller pair.
That is, if the apparatus has higher separation property, the conveyance roller pair is worn out early due to increased pulling load at the conveyance roller pair, and to the contrary, if the pulling load is reduced at the conveyance roller pair, the sheet separation property may be sacrificed.
This invention is conceived in consideration to the above problems. It is an object of the invention to provide a sheet feeding apparatus, as well as an image forming apparatus and an image reading apparatus having the sheet feeding apparatus, in which durability of conveying means and high separation function are compatible by performing stable feeding and separation operation with a very simple structure and by reducing a load exerted to a conveying means placed on a downstream side of a feeding means in a sheet feeding direction. The sheet feeding apparatus includes sheet supporting means for supporting sheets, feeding means for feeding in a sheet feeding direction the sheets fed from the sheet supporting means, and separating means for rotating in a direction opposite to the sheet feeding direction to separate the sheets sheet by sheet in pressurized contact with the feeding means, wherein the circumferential speed of the separating roller is set lower than the circumferential speed of the feeding roller.